1. Field of the Invention
This invention relates to monitoring the cleaning effectiveness of cleaning systems and methods in which vibrations (e.g., vibrations having a megasonic or ultrasonic frequency) are applied to a volume of fluid in which an object to be cleaned is at least partially immersed.
2. Related Art
It is sometimes necessary or desirable to clean particles (particularly small particles) from the surface of an object. For example, during processing of a semiconductor substrate (e.g., a semiconductor wafer) to form a device incorporating that substrate (e.g., an integrated circuit chip), it is typically necessary to clean the surfaces of the substrate to remove particles and other contamination on the substrate surfaces. It has been demonstrated that the surfaces of an object such as a semiconductor substrate can be cleaned by applying vibrations to a volume of fluid in which the object is immersed. In particular, vibrations having a megasonic or ultrasonic frequency have been used in cleaning systems and methods of this type. Such cleaning systems are well known and are described in, for example, Ahmed A. Busnaina et al., "An Experimental Study of Megasonic Cleaning of Silicon Wafers," J. Electrochem. Soc., Vol. 142, No. 8, August 1995, pp. 2812-2817; C. J. Gow et al., "A Method for Evaluating Cleaning Techniques for the Removal of Particulates from Semiconductor Surfaces," Proceedings of the 2nd International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, 1992, pp. 366-371; and Glenn Gale et al., "How to Accomplish Effective Megasonic Particle Removal," Semiconductor International, August 1996, pp. 133-136 and page 138, the disclosures of which are incorporated by reference herein.
FIG. 1 is a perspective view of part of a typical dump rinser 100 that can be used to clean an object by vibrating a fluid in which the object is immersed. One or more substrates 101 (multiple substrates are shown in FIG. 1, though this need not necessarily be the case) are positioned in a substrate carrier 102. The substrates 101 can be, for example, semiconductor wafers. The substrate carrier 102 is positioned within a tank 103 which is filled with water (e.g., deionized water), such that the substrates 101 are immersed in the water. As explained further below with respect to FIG. 2, one or more transducers 104 are excited at a predetermined frequency (such as a megasonic or ultrasonic frequency) to produce vibrations that cause pressure waves (shown pictorially in FIG. 1 by the dark arrows) in the water that effect removal of particles from the surfaces of the substrates 101.
FIG. 2 is a block diagram of a typical megasonic dump rinser 200. A system controller 201 (e.g., any conventional digital computing device including a mechanism for operator input and a display) enables an operator to input instructions for operation of the megasonic dump rinser 200. An electrical signal from a power supply 202 is input to each of two conventional RF generators 204a and 204b. Each of the RF generators 204a and 204b convert the corresponding electrical signal to an electrical signal having a radio frequency in accordance with instructions transmitted from the system controller 201 to the RF generator 204a or 204b. The electrical signal output from each of the RF generators 204a and 204b is input to a corresponding conventional balun transformer 205a or 205b. Each of the balun transformers 205a and 205b impedance matches the input signal to enable production of an electrical signal from the balun transformer 205a or 205b having maximum power. The electrical signal output from each of the balun transformers 205a and 205b is input to a corresponding transducer 206a or 206b. Each of the transducers 206a and 206b transforms the input electrical signal into a mechanical signal (i.e., vibrations) having the same frequency as the input electrical signal. (Herein, "radio frequency" may be used when referring to an electrical signal having such frequency, while "megasonic frequency" may be used when referring to a mechanical quantity, such as vibrations, having such frequency.) The transducers 206a and 206b, which operate in a manner similar to an audio speaker, may each be implemented by, for example, a plate including piezoelectric elements, vibration of the plate being produced in response to input of the electrical signal to the piezoelectric elements. Though two sets of an RF generator, balun transformer and transducer are shown in FIG. 2, one, three or more sets can be used.
The transducers 206a and 206b are positioned in a tank 207 which is filled with water 208, such that the transducers 206a and 206b are immersed (at least partly and typically entirely) in the water 208. One or more substrates 203 are also positioned in the tank 207 so that the substrates 203 are immersed (at least partly and usually entirely) in the water 208. The vibrations of the transducers 206a and 206b produce pressure waves in the water 208, resulting in alternating periods (which occur at the same frequency as that of the vibrations) of high and low pressure at the surfaces of the substrates 203. The agitation produced at the substrate surfaces by the pressure waves causes particles on the substrate surfaces to be dislodged, thereby cleaning the substrate surfaces.
During periods of low pressure, bubbles can be formed by cavitation (i.e., cavities are formed which fill with vapor or gas) on surfaces in the water 208 that are perpendicular to the direction of motion of the pressure wave. A subsequent high pressure part of the pressure wave can cause the bubbles to collapse. The increased agitation produced at such surfaces by the formation and collapse of bubbles aids in dislodgement of particles from those surfaces. Thus, the substrates 203 are preferably positioned in the water 208 such that the substrate surfaces to be cleaned are perpendicular (or substantially perpendicular) to the direction of motion of the pressure wave, so that the cleaning of the substrate surfaces can be enhanced by the above-described bubble formation and collapse.
Previously, cleaning effectiveness during a megasonic cleaning process has been monitored by monitoring the power output from the RF generator(s), it being assumed that cleaning effectiveness increases directly with the magnitude of RF generator power output. If the RF generator power output remains above a specified magnitude, cleaning is deemed to be adequate; if the power output decreases below the specified magnitude, cleaning is considered to be inadequate.
However, the magnitude of the RF generator power output may not be an accurate indicator of cleaning effectiveness. For example, the installation of the mechanical parts of a megasonic cleaning system (e.g., the mounting of the tank that holds the cleaning fluid), the physical characteristics of the electrical cabling of a megasonic cleaning system, or degradation of the operation of an aspect of a megasonic cleaning system (e.g., degradation of transducer operation over time, such as may occur, for example, if a transducer mount loosens or an electrical short occurs in a transducer) may change the relationship between RF generator power output and cleaning effectiveness, such that maximum cleaning effectiveness is obtained at other than the maximum RF generator power output or such that inadequate cleaning effectiveness (though the maximum cleaning effectiveness) is obtained at the maximum RF generator power output. Moreover, even if maximum and/or adequate cleaning effectiveness is initially obtained at maximum RF generator power output, over time this relationship may not hold; in particular, cleaning effectiveness may degrade even though a corresponding decrease in RF generator power output has not occurred. Thus, monitoring RF generator power output may not be an adequate means of monitoring the cleaning effectiveness of a megasonic cleaning system.
The effectiveness of megasonic cleaning is believed to be related to the magnitudes of the frequency and amplitude of the pressure wave induced in the cleaning fluid (e.g., water), and, if cavitation occurs, the size (which is related to the amplitude of the pressure wave) and frequency of formation (which is related to the frequency of the pressure wave) of the bubbles formed on surfaces of the objects being cleaned. Currently, there is no way to directly monitor these quantities during operation of a megasonic cleaning system. The ability to monitor the efficacy of a megasonic cleaning process or system would be greatly enhanced by the capability to monitor these quantities. More generally, the capability to monitor such quantities can be useful in monitoring cleaning effectiveness in any cleaning system or method in which vibrations are applied to a volume of fluid in which an object to be cleaned is at least partially immersed.